1. Technical Field
The present invention pertains to synchronization of nodes within a network. In particular, the present invention pertains to a time-of-day synchronization (TOD) within wireless ad-hoc networks that determines an appropriate time for network nodes to switch to a common GPS time without disruption of existing communication links.
2. Discussion of Related Art
Commercial wireless devices employ clock synchronization techniques for their operation in WPAN (Wireless Personal Area Networks) and WLAN (Wireless Local Area Networks) type networks. Exemplary WPAN type devices include Bluetooth devices designed to support low bandwidth and short distance (e.g., less than ten meters) wireless connections. Bluetooth specifies the protocols to be used by different handheld computing devices in order to communicate and exchange data. For timing purposes, Bluetooth specifies a master/slave clock synchronization mechanism allowing synchronization between neighboring nodes that reside one-hop away from each other within the network.
The IEEE 802.11 WLAN standard specifies two different approaches for time synchronization. One approach accommodates infrastructure-based networks, while the other approach accommodates independent networks. With respect to infrastructure-based networks, IEEE 802.11 provides a master/slave clock synchronization mechanism, where a special fixed node (referred to as an access point (AP)) is used as a master. In an independent network, a mobile node transmits a beacon message at a selected beacon period. Each node receiving the beacon message updates the node clock with the value in the received beacon message in response to the received value being greater than the node current local time. If the received value is less than the node local time, the received value is discarded.
In a tactical environment, Radio Frequency (RF) communications must be protected against major enemy threats including jamming and signal interception. In order to protect against these threats, a modern tactical system constantly changes the transmission frequency (e.g., by frequency hopping (FH), by direct sequence spread spectrum (DSSS), by both techniques, etc.) and encrypts the transmitted data with a time dependent encryption algorithm. Tactical network nodes must therefore be accurately synchronized in time to communicate. The time synchronization problem is especially difficult in a tactical network due to the volatility of RF links.
An example of a tactical ad-hoc radio system employing a conventional approach to time-of-day (TOD) synchronization includes a Near Term Data Radio (NTDR) system from ITT. This system employs three time-of-day (TOD) message types (e.g., Cold Start, Late Net Entry (LNE) and In-Net) to enable all radios or nodes within the network to synchronize with a common time (e.g., time-of-day). In Cold Start (CS) mode, all radios or nodes within the network use a fixed time (Transmission Security (TRANSEC)). This enables all radios within the network to listen to the Cold Start messages without an initial time (e.g., time-of-day) reference. Upon receiving a Cold Start time-of-day update message, a radio extracts the transmitter time from the message and uses that time to update the node time-of-day. Subsequently, the radio in Cold Start mode enters a Late Net Entry (LNE) mode and selects an LNE time (TRANSEC), which is generally within six minutes from an In-Net time-of-day. A radio dwelling in LNE mode remains in that mode until the radio time-of-day is within twenty milliseconds (20 msec) of the transmitter time. Once this occurs, the radio transitions into an In-Net mode. An In-Net mode message is used for normal message transmission within the network.
The time-of-day synchronization scheme of the Near Term Data Radio (NTDR) system has been modified to satisfy the constrained requirements of a Small Unit Operation (SUO) network. This type of network includes extremely mobile, volatile, power and bandwidth limited operational conditions. The basic functionality of the Cold Start (CS), Late Net Entry (LNE) and In-Net modes of the Near Term Data Radio (NTDR) system are similar to “Isolated”, “In Sync”, and “Associated” modes employed in the SUO system. Due to the severe timing constraints imposed by the Small Unit Operation (SUO) security features, nodes must establish a common network time before commencing communications. This is accomplished by the time-of-day (TOD) synchronization scheme or protocol. If a node with local Global Positioning System (GPS) time or capability is present, the aggregate network time (e.g., “net time” or non-GPS based network time) must be slowly pulled towards the GPS time in order to facilitate multi-tier operation. However, the time-of-day synchronized nodes constantly lose and regain connectivity with each other in a tactical or ad-hoc environment. In order to solve the problem of fragmented networks, a “flywheel” technique has been proposed to allow nodes to continue to predict, and therefore track, the movement of the network time as adjusted by neighboring nodes. This type of technique or scheme is disclosed in U.S. Patent Application Publication No. 2004/0047307 (Yoon et al.), the disclosure of which is incorporated herein by reference in its entirety. One of the advantages of the flywheel time-of-day synchronization scheme includes a roaming node with local GPS time being able to synchronize with existing (non-GPS based) network time immediately, and slowly pull the network time toward the GPS based time.
Although the flywheel technique overcomes difficulties encountered with respect to time synchronization within an ad-hoc environment, the technique can stand some improvement. For example, when the network time (e.g., non-GPS based time) differs from the GPS time by one-second, the interval for the network time to reach the GPS based time is in excess of one hour (e.g., 1.33 hours). The time-offset between the network time and the GPS time is unpredictable and depends upon the operational scenario (e.g., the node starting first and/or the node having a local GPS time among the neighboring nodes, the start time of the node, etc.). This potentially long pulling time by the flywheel technique may be unacceptable in the highly mobile, volatile, power and bandwidth limited tactical networks. In an attempt to overcome this problem, a node with local GPS time may flood an LNE message to neighboring nodes directing the nodes to acquire the GPS signal and set node clocks to GPS time immediately. However, the sudden changes of network time may disturb the current communication in the existing network since nodes more than one hop away from a group or island head node may not timely receive information related to the GPS transition and be unaware of when to perform that transition to GPS time.
Further, if a node local clock time is not updated by neighboring nodes with GPS time, drifts in the local clock may accumulate during the long pulling period of the flywheel technique. The accumulated clock drifts during the flywheel pulling interval may cause fragmented networks. For example, a node with a clock drift rate of one part per million (ppm) that updates the node clock via the flywheel technique loses connectivity with the existing network after two-hundred updates since the maximum value for staying within the network time is exceeded (e.g., Δtmax of two-hundred microseconds (200 μsec)).
When two unconnected islands or groups of nodes (e.g., fragmented networks) come within proximity of one another, some of the member nodes within the different islands begin to establish RF neighbor relationships in order to merge. Unless time synchronization is achieved between these neighboring nodes, the merging of the fragmented networks cannot be resolved.
The time synchronization between fragmented networks is very expensive with respect to the hardware/software processing required. For example, an Associated node within the SUO system requires an extra receiver during the pulling period of the flywheel technique in order to monitor reception of an LNE message for resolving fragmented networks for merging. Even if the node receives the LNE message, additional processing or techniques are needed to arrange other nodes and set the node times to the targeted time without interrupting the communication links.
Thus, pulling network time to GPS time by a node with GPS may be problematic in tactical ad-hoc networks due to a potentially long pulling time, accumulated clock drifts and very expensive hardware/software implementation for a merge.